Gate drive technique for a bidirectional blocking lateral MOSFET

ABSTRACT

A gate drive circuit for a bidirectional blocking MOSFET, the bidirectional blocking MOSFET being characterized in the source region is not shorted to the body region. In one embodiment, the gate drive circuit includes diodes connected between the source/drain regions and a charge pump, the charge pump generating a gate drive voltage applied to a gate of the bidirectional blocking MOSFET. In a second embodiment, a charge pump generates a gate drive voltage which is applied to the gate of the bidirectional blocking MOSFET, and is also connected to the source/drain regions through zener diodes. In the second embodiment, the potential applied to the gate of the bidirectional blocking MOSFET is limited to a zener diode drop above the lower of the voltages of the source/drain regions. In a fourth embodiment, a charge pump generates a floating gate drive voltage which is applied to gate of the bidirectional blocking MOSFET through first and second depletion mode MOSFETS. In the fourth embodiment, the gate drive voltage is limited to the threshold level of the first and second depletion mode MOSFETs and the voltage present on the more negative of the source/drain regions. In the second and fourth embodiments, the limited gate drive allows for a minimal gate oxide thickness, thereby improving switch resistance.

This application is a continuation of application Ser. No. 08/160,560,filed Nov. 30, 1993, now U.S. Pat. No. 5,510,747.

CROSS REFERENCE TO RELATED APPLICATION

This application is related to application Ser. No. 08/159,900 (now U.S.Pat. No. 5,536,977) and application Ser. No. 08/160,539 (now U.S. Pat.No. 5,420,451), both of which were filed on the same data as the parentapplication, and both of which are incorporated herein by reference.

FIELD OF THE INVENTION

Users of battery-powered devices such as notebook computers require thatthe devices be usable for long periods of time between batteryrecharges. This requirement has led to cascaded battery arrangements, inwhich a primary battery, a secondary battery, etc., are connected to thedevice in succession. Frequently an AC/DC converter is also provided toallow the user to conserve battery power when he is near a source of ACpower. A connection for an external backup battery may also be provided.

Such an arrangement is illustrated in FIG. 1 wherein a primary batteryB1 and a secondary battery B2 are connected via switches S1 and S2,respectively, to a load L, which could be a DC/DC converter supplying,for example, a notebook computer. The supply connections are madethrough a bus which is designated B.

Also connected to bus B is an AC/DC converter C3 which supplies powerthrough a switch S3. The voltage supplied by primary battery B1 isdesignated V₁, the voltage supplied by secondary battery B2 isdesignated V₂, and the voltage supplied by AC/DC converter C3 isdesignated V₃. A backup battery B4 is also connected to bus B.

In the operation of this multiple battery arrangement, only one ofswitches S1, S2, and S3 would normally be closed at any given time. Theremaining switches would be open. When power is supplied by primarybattery B1, for example, switch S1 is closed and switches S2 and S3 areopen.

As the power sources are switched in and out, the voltage acrossswitches S1, S2, and S3 can vary both in magnitude and direction. Thisis illustrated in FIGS. 2A-2C. As shown in FIG. 2A, for example, theoutput V₂ of battery B2 might be 14 V at a given point in time. Ifbattery B2 is then supplying power, the voltage V_(bus) would also equal14 V. If battery B1 is fully charged, its output voltage V₁ might be 18V. In this case, the left side of switch S1 would be positively charged.On the other hand, assume the same situation except that battery B1 isdischarged, so that V₁ is 6 V. In this case, the right side of switch S1is positively charged, as shown in FIG. 2B. A third alternative isillustrated in FIG. 2C where battery B1 is discharged, battery B2 isfully charged, and bus B is supplied by AC/DC converter C1. In theexample, V₁ is shown as equalling 6 V, V₂ is shown as equalling 17 V,and V₃ is shown as equalling 12 V. In this case, the right side ofswitch S1 is positively charged, and the left side of switch S2 ispositively charged.

In summary, any of switches S1-S3 may have to withstand a voltage ineither direction. The only thing known for certain is that all of thevoltages applied to these switches will be above ground.

The device may also be equipped with an internal battery charger, asillustrated in FIG. 3. A battery charger C5 is connected to battery B1via a switch S4 and to battery B2 via a switch S5. Battery charger C5may be supplied from the output of AC/DC converter C3 or (optionally)directly from the power main. As illustrated in FIG. 4, battery chargerC5 may deliver a voltage as high as 24 V for quick battery charging. Inthe condition illustrated in FIG. 4, battery B2 is being charged, andthe V₁ output of battery B1 is equal to 12 volts. Switch S4 thereforemust withstand a voltage difference of 12 V. However, since deepdischarging of a rechargeable battery is known to extend its life, V₁could drop to below 6 V, in which case switch S4 would need to withstandover 18 V, with its left side being positively charged. On the otherhand, when battery charger C5 is not operative it may have a shorted orleaky characteristic, and switches S4 and S5 would then have to blockvoltages in the other direction. Therefore, switches S4 and S5 must alsobe bidirectional current blocking.

The foregoing would not represent a problem if switches S1-S5 weremechanical switches. However, it is preferable to use semiconductortechnology, and in particular MOSFET technology, in fabricating theseswitches. Power MOSFETs are typically fabricated with a source-bodyshort to ensure that the intrinsic bipolar transistor (represented bythe source, body and drain regions) remains turned off at all times. Theprior art teaches generally that a good source-body short is fundamentalto reliable parasitic-bipolar-free power MOSFET operation.

The use of a source-body short has the effect of creating a diode acrossthe drain and body terminals of the MOSFET which is electrically inparallel with the MOSFET. For a P-channel device, the cathode of thediode is connected to the drain; for an N-channel device, the anode ofthe diode is connected to the drain. Thus, a MOSFET must never beexposed to voltages at its source-body and drain terminals which wouldcause the "antiparallel" diode to become forward-biased. FIGS. 5A-5Dillustrate the polarity of the antiparallel diode (shown in hatchedlines) for a vertical N-channel DMOS device (FIG. 5A), a verticalP-channel DMOS device (FIG. 5B), a lateral N-channel device (FIG. 5C),and a lateral N-channel DMOS device (FIG. 5D).

Accordingly, conventional MOSFETs are not suitable for switches S1-S5because they are not capable of blocking bidirectional currents. InFIGS. 2A-2C, for example, the antiparallel diodes across switches S1 andS2 are shown in hatched lines, with their anode and cathode terminalsarranged so as would be required to block the flow of current throughthe switches. If the polarity of the voltages across the switches werereversed, the antiparallel diodes would become forward-biased.

One possible solution to this problem would be to connect two MOSFETs ina back-to-back arrangement, as illustrated schematically in FIGS. 6A-6C.FIG. 6A illustrates a pair of NMOS devices having a common source, FIG.6B illustrates a pair of NMOS devices having a common drain, and FIG. 6Cillustrates a pair of PMOS devices having a common source. Theseback-to-back arrangements double the on-resistance of the switches,however, and therefore detract significantly from the amount of powerdelivered to the computer or other device.

Accordingly, what is needed is a bidirectional current blocking MOSFETwhich has the on-resistance of a normal MOSFET and yet does not containan antiparallel diode across the drain and body terminals.

In addition, what is needed is a gate drive circuit for thebidirectional blocking MOSFET which allows bidirectional current flow.

SUMMARY OF THE INVENTION

In accordance with this invention, a gate drive circuit is provided fora bidirectional blocking MOSFET. The bidirectional blocking MOSFETincludes first and second regions of a first conductivity type separatedby a channel region of a second conductivity type, the first and secondregions serving as a source and drain of the MOSFET. The bidirectionalblocking MOSFET also includes a body region, a gate and a gate oxidelayer between the body region and the gate, and is characterized in thatneither of the first and second regions are shorted to the body region,and voltages that are applied to the first and second regions are botheither higher than or lower than a voltage at which the body region ismaintained, thereby preventing forward-biasing of the junctions betweenthe body and the first and second regions.

The gate drive circuit generates a gate voltage which is limited by amaximum voltage supported by the gate oxide layer and is determined bythe lowest voltage of the first and second regions.

In accordance with a first embodiment of the present invention, a gatedrive circuit for a bidirectional blocking MOSFET includes a first diodehaving an anode connected to the first region, and a second diode havingan anode connected to the second region, a charge pump connected to thecathodes of the first and second diodes, the charge pump generating agate drive voltage applied to the gate of the bidirectional blockingMOSFET. A grounded zener diode is connected to limit the gate drivingvoltage to a predetermined maximum value.

In accordance with the first embodiment of the present invention, thegate drive voltage is determined by the higher of the voltages of thefirst and second regions and is fixed relative to ground. This assuresthat the gate drive voltage is sufficient to allow bidirectional currentflow regardless of the relative voltages on the first and secondregions.

In accordance with a second embodiment of the present invention, acharge pump generates a gate drive voltage which is applied to the gateof the bidirectional blocking MOSFET, and is also applied to the firstand second regions through a first zener diode having a cathodeconnected to the gate drive voltage and an anode connected to the firstregion, and a second zener diode having a cathode connected to the gatedrive voltage and an anode connected to the second region. In addition,first and second diodes are connected between the first and secondregions and the charge pump.

In accordance with the second embodiment, the potential applied to thegate of the bidirectional blocking MOSFET is limited to a zener diodedrop above the lower of the voltages of the first and second regions.This produces a "floating" gate drive voltage which reduces thenecessary thickness of the gate oxide layer separating the gate from thefirst and second regions.

In accordance with a third embodiment of the present invention, a chargepump generates a gate drive voltage which is applied to the gate of thebidirectional blocking MOSFET through a P-channel MOSFET, and is alsoapplied to the first and second regions through a current source, whichis connected to a first zener diode having a cathode connected to thegate drive voltage and an anode connected to the first region and asecond zener diode having a cathode connected to the gate drive voltageand an anode connected to the second region. The cathodes of the firstand second zener diodes are connected to the gate of the P-channelMOSFET. In addition, first and second diodes are connected between thefirst and second regions and the charge pump. An optional N-channelMOSFET is connected to the gate of the bidirectional MOSFET and connectsthe gate to ground when the charge pump is disconnected from theP-channel MOSFET.

In accordance with the third embodiment, the potential applied to thegate of the bidirectional blocking MOSFET is limited by the lower of thevoltages of the first and second regions through the P-channel MOSFET.This produces a "floating" gate drive voltage, similar to theabove-described second embodiment, in which the load on the charge pumpis reduced due to the current source. Further, the grounded N-channelMOSFET is turned on when the charge pump is disconnected from thebidirectional blocking MOSFET, thereby connecting the gate of thebidirectional blocking MOSFET to ground.

In accordance with a fourth embodiment of the present invention, acharge pump generates a floating gate drive voltage which is applied tothe gate of the bidirectional blocking MOSFET through first and seconddepletion mode MOSFETS. The drain of the first depletion mode MOSFET isconnected to the charge pump through a switch. The drain of the seconddepletion mode MOSFET is connected to the source of the first MOSFET,and the source of the second MOSFET is connected to the gate of thebidirectional blocking MOSFET. The gate of the first depletion modeMOSFET is connected to the second region and the gate of the seconddepletion mode MOSFET is connected to the first region. An optionalN-channel MOSFET is connected to the gate of the bidirectional MOSFETand connects the gate to ground when the charge pump is disconnectedfrom the first depletion mode MOSFET.

In accordance with a fifth embodiment of the present invention, aP-channel MOSFET is connected between the charge pump and thebidirectional blocking MOSFET, and first and second depletion modeMOSFETS are connected between the charge pump and the gate of theP-channel MOSFET. The drain of the first depletion mode MOSFET isconnected to the charge pump through a switch. The drain of the seconddepletion mode MOSFET is connected to the source of the first MOSFET,and the source of the second MOSFET is connected to the gate of theP-channel MOSFET. The gate of the first depletion mode MOSFET isconnected to the second region and the gate of the second depletion modeMOSFET is connected to the first region. An optional N-channel MOSFET isconnected to the gate of the bidirectional MOSFET and is turned on whenthe charge pump is disconnected from the first depletion mode MOSFET.

In accordance with the fourth and fifth embodiments, the gate drivevoltage is limited to the threshold level of the first and seconddepletion mode MOSFETs and the voltage present on the more negative ofthe first and second regions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a schematic drawing of a multiple battery powersupply arrangement, including disconnect switches.

FIGS. 2A, 2B and 2C illustrate possible voltage differences illustratedby the disconnect switches shown in FIG. 1.

FIG. 3 illustrates a schematic diagram of a multiple battery powersupply arrangement, including a battery charger.

FIG. 4 illustrates possible voltage differences experienced by thedisconnect switches shown in FIG. 3.

FIGS. 5A-5D illustrate, respectively, a vertical N-channeldouble-diffused MOSFET, a vertical P-channel double-diffused MOSFET, alateral N-channel MOSFET, and a lateral N-channel double-diffusedMOSFET, all of which contain a source-body short.

FIGS. 6A-6C illustrate bidirectional current blocking switchesconsisting of back-to-back MOSFETs.

FIG. 7 illustrates a multiple battery power supply arrangementcontaining bidirectional blocking MOSFET switches in accordance with theinvention.

FIG. 8 illustrates a non-drifted bidirectional blocking MOSFET.

FIG. 9 illustrates a drifted bidirectional blocking MOSFET.

FIGS. 10A and 10B illustrate a non-drifted bidirectional blocking MOSFETin an off condition.

FIGS. 11A-11C illustrate the relationship between the gate, source anddrain regions of a bidirectional blocking MOSFET in an off condition, atturn-on, and after turn-on.

FIGS. 12A and 12B illustrate the gate-to-source and source voltage of abidirectional blocking MOSFET when a fixed gate voltage is applied.

FIGS. 13A and 13B illustrate the gate-to-source and source voltages of abidirectional blocking MOSFET when a floating gate voltage is applied.

FIG. 14 illustrates a charge pump circuit used in the gate drive circuitof the present invention.

FIG. 15 illustrates a first embodiment of the gate drive circuit inaccordance with the present invention.

FIG. 16A illustrates a second embodiment of the gate drive circuit inaccordance with the present invention.

FIG. 16B illustrates a third embodiment of the gate drive circuit inaccordance with the present invention.

FIG. 17A illustrates a fourth embodiment of the gate drive circuit inaccordance with the present invention.

FIG. 17B illustrates a fifth embodiment of the gate drive circuit inaccordance with the present invention.

DESCRIPTION OF THE INVENTION Bidirectional Blocking MOSFET Switches

FIGS. 7-9 illustrate bidirectional blocking MOSFET switches associatedwith the gate drive circuit of the present invention. A briefdescription of the bidirectional blocking MOSFETs is herein provided forreference. Additional detail regarding the structure and operation ofthe bidirectional blocking MOSFETs is provided in copending U.S.application Ser. No. 08/159,900, now U.S. Pat. No. 5,536,977, issuedJul. 16, 1996, which is incorporated herein in its entirety.

FIG. 7 shows a multiple source power supply arrangement illustrating atypical use for bidirectional current blocking MOSFET switches. Switch70 is connected to a battery 72, which supplies a voltage V₁, and switch71 is connected to an AC/DC converter 73 which supplies a voltage V₂.While two power sources and two switches are illustrated in FIG. 7, itwill be apparent that any number of batteries or other power sourcescould be included in the arrangement. Switches 70 and 71 connect into abus 74, which supplies an output voltage V_(out) to a load (not shown).

FIG. 8 illustrates the bidirectional current blocking MOSFET switch 70of FIG. 7 in greater detail. Switch 70 is a lateral MOSFET which isillustrated in cross section. Switch 70 is preferably formed in either astripe or cellular pattern in a substrate 75, which in this embodimentis shown as including P-type semiconductor material. N+ regions 76 and77 are formed at the surface of P substrate 75, separated by a channelregion 78. In the description below, the regions 76 and 77 arearbitrarily referred to as source and drain regions, although neitherregion 76 nor region 77 is technically a source or a drain because themost positive voltage cannot be determined except during operation. Agate 79 is formed over channel region 78, separated from channel region78 by an oxide layer 80. It will be noted that switch 70 is asymmetrical device, and regions 76 and 77 are not referred to as sourceor drain regions, since either of them can be biased positively ornegatively. A terminal 81 connects N+ region 76 to battery 72 and aterminal 82 connects N+ region 77 to bus 74. Gate 79 is supplied by agate voltage V_(g) from a gate drive circuit according to the presentinvention, which is described below. Finally, the junction between thegrounded P substrate 75 and N+ region 76 is represented by a diode D₁and the junction between the grounded P substrate 75 and N+ region 77 isrepresented by a diode D₂.

FIG. 9 illustrates a drifted bidirectional current blocking switch 90,which can be used in place of the non-drifted switch 70 of FIG. 8 inhigh voltage (greater than about 14 V) applications. A switch 90contains N-drift regions 92 and 93, which, in the off-state, serve tolimit the strength of the electric field between the gate 95 and thediffused N+ source/drain regions and to improve the junction breakdownvoltage across oxide region 94. Since the gate must be biasedsufficiently positive relative to ground to allow the device to conductover the specified operating range, the oxide layer 94 separating thegate 95 from the channel region 96 must be thick enough to accommodatethe maximum gate voltage reliably. Since either side of switch 90 mayserve as the "drain" in a given situation, a drift region must beprovided on both sides of the channel region.

Each of the drifted and non-drifted bidirectional blocking MOSFETS 70and 90, discussed above, is symmetrical; that is, neither the drain northe source is grounded to the body. The undrifted device 70 is ingeneral useful only up to about 14 volts because higher voltages subjectthe gate oxide 80 to high fields and may result in avalanche breakdownunder the gate 79. The drifted device 90 may be used up to 18 andpossibly to 26 volts (or even higher) depending on its design.

Design Optimization of Gate Drive Circuits for Bidirectional MOSFETS

FIGS. 10A and 10B illustrate a fundamental restriction of theabove-described non-drifted device 70. As indicated in FIG. 10A, in theoff-state, the gate 79 and body 75 are both grounded. The terminal 81which is connected to the battery 72, is biased at some voltageV_(batt). The other terminal 82 is biased to V_(bus). As indicated inFIG. 10B, since the gate 79 overlaps the heavily-doped regions 76 and77, virtually the entire applied bias (the difference between V_(bus)and V_(bat)) must be supported across the gate oxide layer 80 during theoff condition. For an 8V bias using a 50% derating, the thickness of theoxide layer 80 must exceed 160 Å to guarantee a 16V oxide rupturevoltage.

It is commonly known that the thickness of the gate oxide layer neededto assure proper operation of a MOSFET depends on three variables:

1. The MOSFET's threshold voltage (gate-to-drain voltage), which istypically in the range of 0.7 to 2.5 volts;

2. The degree of overdrive beyond the threshold voltage which isnecessary to achieve a desired minimum on-resistance, which is typically4 to 10 volts above threshold voltage; and

3. Whether the gate drive circuit provides a gate drive voltage fixedrelative to ground, or a gate drive voltage which floats with the sourcevoltage (i.e., the voltage on the least positively biased diffusedregion).

FIGS. 11A-11C illustrate the effect of a fixed 8V gate drive voltage ona non-drifted bidirectional blocking MOSFET driven by a 5V battery 1103.As shown in FIG. 11A, in an off condition, the gate 1101 is connected toground, the drain 1102 is connected to the 5 volt battery 1103, and thesource 1104 is connected to a load 1105, which is represented as acapacitor. As shown in FIG. 11B, at a time t₁ of turn-on, a gate drivevoltage of 8V is applied to the gate 1101, and the gate-to-sourcevoltage V_(gs) across the gate oxide 1106 becomes 8V. This producescurrent flow from the drain 1102 to the source 1104 with low resistancedue to the high V_(gs). As shown in FIG. 11C, at a time t₂ afterturn-on, the load voltage reaches 5V, thereby reducing the gate tosource voltage V_(gs) to 3V, which in turn increases the on-resistanceof the MOSFET 1100. In other words, by limiting the gate drive voltageto the system power supply and referencing it to ground, thegate-to-source voltage V_(gs) decreases as the load voltage increases,thereby increasing the on-resistance of the MOSFET 1100 during a turn-ontransition.

FIGS. 12A and 12B illustrate one method for reducing the on-resistanceof a non-drifted bidirectional blocking MOSFET 1200. As indicated inFIG. 12A, if a fixed gate voltage V_(g) of 15V is applied to the gate1201 of the MOSFET 1200, a 10V gate-to-source voltage V_(gs) iseventually generated at a time t₂ after turn-on. Because of the higherpotential, a minimal on-resistance is created between the source 1204and the drain 1202. However, because a fixed V_(g) is used, a V_(gs)potential of 15V is present across the gate oxide layer 1206 separatingthe gate 1201 and the source 1204 at the moment of turn-on t₁. That is,the entire 15V is necessarily supported across the gate oxide layer1206. Using a 50% derating, the 15V V_(gs) value mandates the gate oxidelayer 1206 to be over 300 Å.

FIGS. 13A and 13B illustrate another method for improving theon-resistance of a non-drifted bidirectional blocking MOSFET 1300 inwhich the gate voltage V_(g) is referenced to the source 1304 and floatswith the load voltage. In such a case, V_(gs) remains 10 volts at alltimes, thereby eliminating the need for a gate oxide layer 1306 which iscapable of supporting a higher voltage. The gate-to-ground referencevalue of the gate voltage V_(g) for the floating gate drive circuit canbe expressed as the source voltage V_(s) plus 10 volts.

Note that the gate oxide layer 1306 never supports more than 10 voltsbecause the formation of the inversion layer shields the gate oxidelayer 1306 from the substrate potential. Therefore, if one region 1302or 1304 is always more positive during conduction, then the gate drivercircuit can be permanently referenced to the more negative source side.The combination of such a gate drive circuit with the MOSFET 1300 wouldproduce a bidirectionally-blocking, unidirectionally-conducting MOSFET;that is, current through the MOSFET 1300 can only be safely supportedfrom region 1302 to region 1304. If the more positive of the regions1302 and 1304 were referenced, then at the moment of turn-on the gateoxide layer 1306 would have to support both the source voltage and the10 volt gate drive. However, in most cascaded battery applications,discussed above, the more positive region and more negative region arenot known, that is, either region 1302 or 1304 may be the more negativeregion when conduction is desired depending on whether the battery 1307is discharged or newly charged compared to the bus voltage. Therefore,in order to produce a useful floating gate drive voltage V_(g) for thebidirectional blocking MOSFET 1300, it is necessary to reference thegate drive voltage V_(g) to 10 volts above the most negative region 1302or 1304. When this is accomplished, the combination of such a gate drivecircuit with the MOSFET 1300 produces a bidirectionally-blocking,bidirectionally-conducting MOSFET; that is, current through the MOSFETcan be in either direction between regions 1302 and 1304. A gate drivecircuit which produces a floating gate drive voltage V_(g) which is 10volts above the lower region 1302 or 1304 will be disclosed below.

In summary, a non-drifted bilateral blocking MOSFET may be driven by afixed voltage by increasing the thickness of its gate oxide layer, ormay be driven with a floating gate drive voltage which is referenced tothe most negative of the source and drain regions.

The use of a fixed gate drive voltage to drive a drifted bidirectionalblocking MOSFET presents a problem not associated with the non-driftedMOSFET because the thickness of the gate oxide layer in the driftedMOSFET must be too thick to maintain a low body effect. Body effect isthe influence of a reverse-biased source-to-body junction resulting inan increase in threshold voltage V₁ and a commensurate decrease in gatedrive V_(gs) -V₁, thereby leading to an increase in on-resistanceR_(ds). As discussed above, for a MOSFET to achieve a low on-resistance,the fixed gate drive voltage must be 10 volts above the source voltageso that, when the drain voltage and source voltage are equivalent, thegate-to-drain voltage is 10 volts. However, because driftedbidirectional blocking MOSFETs are typically incorporated into systemsdriven by an 18 volt (or higher) battery, and because the gate drivecircuit must produce 10 volts above the system voltage, the gate drivecircuit must produce nearly 30 volts. To assure safe operation and toprevent breakdown, the thickness of the gate oxide layer must be over700 Å in order to support a 60 volt gate-to-drain voltage (using a 50%derating). Like the non-drifted bidirectional blocking MOSFET, the worstcase electric fields appear across the gate at turn-on. After the deviceturns on the inversion layer shields the gate oxide from the substratepotential.

It is interesting to note that the maximum voltage of a symmetricdrifted bidirectional MOSFET is not limited by the off-state conditionsince the drift regions drop most of the gate-to-drain voltage. Insteadthe turn-on and on states are most critical in this voltage range.

Gate Drive Circuits

Gate drive circuits according to the present invention will now bediscussed with reference to FIGS. 14-17.

In each situation discussed above regarding fixed and floating gatedrive voltages, the gate drive voltage for the disclosed drifted ornon-drifted n-channel bidirectional blocking MOSFET must be capable ofexceeding the maximum applied to either terminal region by at least 10volts to achieve minimal on-resistance. The only economically practicalway to provide this voltage is with a charge pump. A charge pump is awell known device used to produce an output voltage V_(cp) which ishigher than its input voltage V_(in). FIG. 14 illustrates one chargepump circuit for reference. Additional discussion regarding charge pumpsis provided in co-owned U.S. application Ser. No. 08/067,365, filed May26, 1993 now U.S. Pat. No. 5,539,610, issued on Jul. 23, 1996, which isincorporated herein in its entirety.

FIG. 15 illustrates a first embodiment of the present invention in whicha fixed gate drive voltage is generated for a bidirectional blockingMOSFET 1500. The gate drive circuit includes a charge pump 1510 havingan input terminal connected to both the source region 1502 and drainregion 1504 of the MOSFET 1500. A first diode D3 is connected betweenthe region 1502 and the charge pump 1510 such that its anode isconnected to the region 1502 and its cathode is connected to the inputterminal of the charge pump 1510. Similarly, a second diode D4 isconnected between the region 1504 and the charge pump 1510 such that itsanode is connected to the region 1504 and its cathode is connected tothe input terminal of the charge pump 1510. The output voltage V_(cp) ofthe charge pump 1510 is connected to a gate 1501 of the MOSFET 1500.Further, a grounded zener diode D5, having a selected breakdown voltageBV_(z), is connected at its cathode to V_(cp). Finally, a switch 1520connected between the gate 1501 and the charge pump 1510.

In the gate drive circuit shown in FIG. 15, the input voltage V_(in) ofthe charge pump 1510 is equal to the voltage on the more positive region1502 or 1504, minus a diode drop associated with diodes D3 and D4. Notethat only the more positive region 1502 or 1504 will forward bias itsassociated diode D3 or D4; the more negative region 1502 or 1504 willnot affect V_(in). For a charge pump circuit similar to that shown inFIG. 14, V_(cp) is typically three times V_(in) minus 2.1 volts.Assuming a 0.7 volt diode drop and a maximum voltage of 10 volts on theregions 1502 and 1504, V_(cp) for the charge pump 1510 would beapproximately 25 volts above ground. After the bidirectional MOSFET 1500is on and the source voltage rises to the drain voltage, V_(gs) becomes15 volts (25 volts minus 10 volts). Beyond a selected zener breakdownvoltage BV_(z), for example 27 volts, the zener diode D5 avalanches andclamps the maximum voltage applied to the gate 1501.

The gate drive circuit in accordance with the first embodiment,described above, generates a gate drive voltage V_(g) which is fixedrelative to ground, and is therefore limited to the above-mentionedlimitations associated with fixed voltage gate drive circuits.

While the diodes D3 and D4 can be eliminated and the charge pump 1510powered from a separate supply voltage, the above-disclosed methodoffers the advantage that the absolute output voltage of the charge pump1510 (relative to ground) increases in proportion to the MOSFET's higherterminal voltage (as long as it is below the zener clamping voltage).For increasing terminal voltages on the MOSFET 1500, the increased gatedrive compensates the increased body effect at higher voltages, therebykeeping the on-resistance R_(ds) low. If the charge pump 1510 werepowered by a fixed input the gate drive may be 13.2 volts, which isinadequate for the full operational range of the MOSFET 1500.

FIGS. 16A and 16B illustrate second and third embodiments of the presentinvention. Elements which are common to both the second and thirdembodiments are identified with the same reference numerals.

FIG. 16A illustrates a gate drive circuit in accordance with a secondembodiment of the present invention in which a floating gate drivevoltage V_(g) is applied to a gate 1601 of a bidirectional blockingMOSFET 1600. As in the first embodiment, the gate drive circuitaccording to the second embodiment of the present invention includes acharge pump 1610 having an input terminal connected to both the sourceregion 1602 and drain region 1604 of the MOSFET 1600. A first diode D3is connected between the region 1602 and the charge pump 1610 such thatits anode is connected to the region 1602 and its cathode is connectedto the input terminal of the charge pump 1610. Similarly, a second diodeD4 is connected between the region 1604 and the charge pump 1610 suchthat its anode is connected to the region 1604 and its cathode isconnected to the input terminal of the charge pump 1610. With thisarrangement, the input voltage V_(in) of the charge pump 1610 is equalto the greater of the voltages V_(x) and V_(y) of the regions 1602 and1604, minus a diode drop associated with diodes D3 and D4. The outputvoltage V_(cp) of the charge pump 1610 is connected to a gate 1601 ofthe MOSFET 1600. Further, a zener diode D6, having a selected breakdownvoltage, is connected at its cathode to V_(cp) and at its anode to theregion 1602. Similarly, a zener diode D7, having a breakdown voltageequal to the breakdown voltage of zener diode D6, is connected at itscathode to V_(cp) and at its anode to the region 1604. Finally, a switch1620 connected between the gate 1601 and the charge pump 1610.

In the gate drive circuit shown in FIG. 16A, the gate drive voltageV_(g) floats at a level equal to the more negative region 1602 or 1604,plus a diode drop associated with zener diodes D6 and D7. That is, thecharge pump output voltage V_(cp) is reduced by the presence of thezener diodes D6 such that the gate drive voltage V_(g) applied to thegate 1601 is clamped at a breakdown voltage associated with the zenerdiodes D6 and D7 above the voltage on the most negative of the regions1602 and 1604. A benefit of the gate drive circuit according to thesecond embodiment is that the gate oxide layer 1606 can be madesubstantially thinner than the gate oxide layer of the above-describedfirst embodiment because the gate-to-source voltage of the secondembodiment is limited to the breakdown voltage of the zener diodes D6and D7. However, one disadvantage of the gate drive circuit of thesecond embodiment is that it can waste battery power because the zenerdiodes D6 and D7 draw current in the breakdown condition.

FIG. 16B illustrates a gate drive circuit in accordance with a thirdembodiment of the present invention. As in the second embodiment, thegate drive, circuit according to the third embodiment includes a chargepump 1610 having an output terminal connected to both the source region1602 and drain region 1604 of the MOSFET 1600 through zener diodes D6and D7. However, in accordance with the third embodiment, a P-channelMOSFET 1630 is connected between the charge pump 1610 and the gate ofthe bidirectional MOSFET 1600, and the gate of the P-channel MOSFET 1630is connected to the zener diodes D6 and D7 such that the gate drivevoltage of the P-channel MOSFET 1630 is determined by the voltage of themore negative region 1602 or 1604. In addition, a current source 1640 isconnected between the charge pump 1610 and the cathodes of the zenerdiodes D6 and D7 to reduce the load driven by the charge pump 1610.

In either of the second and third embodiments, an optional N-channelMOSFET 1650, as shown in FIG. 16B, may be used to pull the gate of thebidirectional MOSFET 1600 to ground when the bidirectional MOSFET 1600is disconnected from the charge pump 1610, such as when the switch 1620is opened.

FIGS. 17A and 17B illustrate fourth and fifth embodiments of the presentinvention. Elements which are common to both the fourth and fifthembodiments are identified with the same reference numerals.

FIG. 17A illustrates a gate drive circuit in accordance with a fourthembodiment of the present invention in which a floating gate drivevoltage V_(g) is applied to a gate 1701 of a bidirectional blockingMOSFET 1700. As in the first through third embodiments, the gate drivecircuit according to the fourth embodiment of the present inventionincludes a charge pump 1710 having an input terminal connected to boththe source region 1702 and drain region 1704 of the MOSFET 1700. A firstdiode D3 is connected between the region 1702 and the charge pump 1710such that its anode is connected to the region 1702 and its cathode isconnected to the input terminal of the charge pump 1710. Similarly, asecond diode D4 is connected between the region 1704 and the charge pump1710 such that its anode is connected to the region 1704 and its cathodeis connected to the input terminal of the charge pump 1710. In addition,a pair of depletion mode MOSFETs M1 and M2 are connected in seriesbetween the charge pump 1710 and the bidirectional blocking MOSFET 1700such that the drain of the depletion mode MOSFET M1 is connected to thecharge pump 1710, the drain of the depletion mode MOSFET M2 is connectedto the source of the MOSFET M1, and the source of the MOSFET M2 isconnected to the gate 1701 of the bidirectional MOSFET 1600. Note thatthe MOSFETs M1 and M2 are "typical" in that they include the traditionalsource-body short. Further, the gate of the MOSFET M1 is connected tothe region 1704 and the gate of the MOSFET M2 is connected to the region1702. Finally, a switch 1720 connected between the drain of the MOSFETM1 and the charge pump 1710.

The gate drive circuit shown in FIG. 17A is similar to theabove-described second and third embodiments in that the gate drivevoltage is limited in proportion to the lower switch voltage. Byselecting the threshold voltages of the MOSFETs M1 and M2 to be -8V, theMOSFETs M1 and M2 will allow their respective sources to rise to avoltage 8V above their gates before shutting off. By connecting thegates to regions 1702 and 1704, the gate voltage Vg is clamped to avalue which is 8V greater than the voltage on the most negative region1702 and 1704. For example, if the region 1702 is maintained at 20volts, and the region 1704 is at ground when Vg is first applied, MOSFETM1 will limit Vg to 8 volts. Note that, without MOSFET M1, MOSFET M2would allow Vg to rise to 8V above 20V, i.e. 28V. The maximum gatevoltage is thereby limited by M1 to 8 volts above the lower voltage. Asthe voltage at region 1704 rises, Vg will track this voltage. Whenregion 1704 is at 10V, Vg is 18 V so that Vgs remains 8 volts.

FIG. 17B illustrates a gate drive circuit in accordance with a fifthembodiment of the present invention. As in the fourth embodiment, thegate drive circuit according to the fifth embodiment includes a pair ofdepletion mode MOSFETs M1 and M2 connected in series with the chargepump 1710. However, in the fifth embodiment, an N-channel MOSFET 1730 isconnected between the charge pump 1710 and the gate of the bidirectionalMOSFET 1700, and the gate of the N-channel MOSFET 1730 is connected tothe source of the depletion mode MOSFET M2.

In either of the fourth and fifth embodiments, an optional N-channelMOSFET 1740, as shown in FIG. 17B, may be used to pull the gate of thebidirectional MOSFET 1700 to ground when the bidirectional MOSFET 1700is disconnected from the charge pump 1710, such as when the switch 1720is opened.

It is noted that the charge pump arrangement taught in the secondthrough fifth embodiments (FIGS. 16A, 16B, 17A and 17B) may receiveinput voltage V_(in) directly from a power supply, instead of throughthe diodes D3 and D4.

The foregoing examples are intended to be illustrative and not limiting.Many additional and alternative embodiments according to this inventionwill be apparent to those skilled in the art. All such embodiments areintended to be covered within the scope of this invention, as defined inthe following claims.

I claim:
 1. A gate drive circuit for generating a gate drive voltage ona gate of a bidirectional MOSFET, the bidirectional MOSFET including abody, first and second regions of a first conductivity type formed inthe body and separated by a channel region of a second conductivity typealso formed in the body, the gate being located over the channel andseparated from the body by a gate oxide layer, the gate drive circuitcomprising:a voltage generating circuit comprising a charge pump havingan input terminal connected to the first region through a firstrectifying device, the input terminal also being connected to the secondregion through a second rectifying device, the voltage generatingcircuit also having an output terminal connected to the gate of thebidirectional MOSFET for limiting the gate drive voltage to a maximumvoltage determined by a thickness of the gate oxide layer.
 2. A gatedrive circuit for generating a gate drive voltage on a gate of abidirectional MOSFET, the bidirectional MOSFET including a body, firstand second regions of a first conductivity type formed in the body andseparated by a channel region of a second conductivity type also formedin the body, the gate being located over the channel and separated fromthe body by a gate oxide layer, wherein the body is maintained at apredetermined voltage, the gate drive circuit comprising:a voltagegenerating circuit comprising a charge pump having an input terminalconnected to the first region through a first rectifying device, theinput terminal also being connected to the second region through asecond rectifying device, the charge pump generating an output voltagewhich is applied to the gate of the bidirectional MOSFET, the outputvoltage being generated solely from current received from the first andsecond rectifying devices, wherein the output voltage from the chargepump produces the gate drive voltage at a level less than or equal to amaximum voltage determined by a thickness of the gate oxide layer.
 3. Agate drive circuit of claim 2 wherein:the first rectifying devicecomprises a first diode having a first terminal connected to the firstregion; the second rectifying device comprises a second diode having afirst terminal connected to the second region, wherein the inputterminal of the charge pump is connected to a second terminal of each ofthe first and second diodes; and the gate drive circuit furthercomprises a grounded zener diode having a first terminal connected tothe output voltage for limiting a level of the output voltage to abreakdown voltage of the zener diode.
 4. A gate drive circuit of claim 3further comprising a switch connected between the charge pump and thegate of the bidirectional MOSFET.
 5. A gate drive circuit of claim 2wherein the voltage generating circuit further comprises:a first zenerdiode having a first terminal connected to the charge pump and a secondterminal connected to the first region; and a second zener diode havinga first terminal connected to the charge pump and a second terminalconnected to the second region.
 6. A gate drive circuit of claim5:wherein the first rectifying device comprises a first diode having afirst terminal connected to the first region; wherein the secondrectifying device comprises a second diode having a first terminalconnected to the second region; and wherein the input terminal of thecharge pump is connected to a second terminal of each of the first andsecond diodes.
 7. A gate drive circuit of claim 5 further comprising asingle pole, double throw switch and a second MOSFET, a first terminalof the switch being connected to an output of the charge pump, a secondterminal of the switch being connected to ground, a common terminal ofthe switch being connected to a gate of the second MOSFET and throughthe second MOSFET to the gate of the bidirectional MOSFET, whereby whenthe switch is thrown so as to disconnect the charge pump from thebidirectional MOSFET, the second MOSFET is turned on to connect the gateof the bidirectional MOSFET to ground.
 8. A gate drive circuit of claim2 wherein the voltage generating circuit further comprises:a secondMOSFET having a first terminal connected to the charge pump and a secondterminal connected to the gate of the bidirectional MOSFET; a firstzener diode having a first terminal connected to a gate of the secondMOSFET and a second terminal connected to the first region; and a secondzener diode having a first terminal connected to the gate of the secondMOSFET and a second terminal connected to the second region.
 9. A gatedrive circuit of claim 8 wherein the voltage generating circuit furthercomprises a current source connected between the charge pump and thegate of the second MOSFET.
 10. A gate drive circuit of claim 8 furthercomprising a switch connected between the charge pump and the gate ofthe bidirectional MOSFET, and a third MOSFET connected between the gateof the bidirectional MOSFET and ground, such that when the switch isopened to disconnect the charge pump from the bidirectional MOSFET, thethird MOSFET is turned on to connect the gate of the bidirectionalMOSFET to ground.
 11. A gate drive circuit of claim 2 wherein thevoltage generating circuit further comprises:a first depletion-modeMOSFET having a first terminal connected to the charge pump and a gateconnected to the second region; and a second depletion-mode MOSFEThaving a first terminal connected to a second terminal of the firstMOSFET, a gate connected to the first region, and a second terminalconnected to the gate of the bidirectional MOSFET.
 12. A gate drivecircuit of claim 11:wherein the first rectifying device comprises afirst diode having a first terminal connected to the first region;wherein the second rectifying device comprises a second diode having afirst terminal connected to the second region; and wherein the inputterminal of the charge pump is connected to a second terminal of each ofthe first and second diodes.
 13. A gate drive circuit of claim 11further comprising a switch connected between the charge pump and thegate of the bidirectional MOSFET, and a third MOSFET connected betweenthe gate of the bidirectional MOSFET and ground, such that when theswitch is opened to disconnect the charge pump from the bidirectionalMOSFET, the third MOSFET is turned on to connect the gate of thebidirectional MOSFET to ground.
 14. A gate drive circuit of claim 2wherein the voltage generating circuit further comprises:a firstdepletion-mode MOSFET having a first terminal connected to the chargepump and a gate connected to the second region; a second depletion-modeMOSFET having a first terminal connected to a second terminal of thefirst depletion-mode MOSFET, a gate connected to the first region; and athird MOSFET having a first terminal connected to the charge pump and asecond terminal connected to the gate of the bidirectional MOSFET, thethird MOSFET also having a gate connected to a second terminal of thesecond depletion-mode MOSFET.
 15. A gate drive circuit of claim14:wherein the first rectifying device comprises a first diode having afirst terminal connected to the first region; wherein the secondrectifying device comprises a second diode having a first terminalconnected to the second region; and wherein the input terminal of thecharge pump is connected to a second terminal of each of the first andsecond diodes.
 16. A gate drive circuit of claim 15 further comprising aswitch connected between the charge pump and the gate of thebidirectional MOSFET, and a fourth MOSFET connected between the gate ofthe bidirectional MOSFET and ground, such that when the switch is openedto disconnect the charge pump from the bidirectional MOSFET, the fourthMOSFET is turned on to connect the gate of the bidirectional MOSFET toground.
 17. A method for generating a gate drive voltage on a gate of abidirectional MOSFET, the bidirectional MOSFET including a body, firstand second regions of a first conductivity type formed in the body andseparated by a channel region of a second conductivity type also formedin the body, the gate being located over the channel and separated fromthe body by a gate oxide layer, wherein the body is maintained at apredetermined voltage potential, the method comprising:connecting thefirst region and the second region to an input terminal of a voltagegenerating circuit, wherein the voltage generating circuit includes acharge pump, through first and second rectifying devices such that thefirst region is connected to the input terminal through a firstrectifying device, and the second region is connected to the inputterminal through a second rectifying device; connecting the gate of thebidirectional MOSFET to an output terminal of the charge pump, whereinthe charge pump generates an output voltage which produces the gatedrive voltage at the gate of the bidirectional MOSFET, the outputvoltage being generated solely from current received from the first andsecond rectifying devices; and limiting the gate drive voltage to alevel less than or equal to a maximum voltage determined by a thicknessof the gate oxide layer.
 18. A method of claim 17:wherein the firstrectifying device is a first diode having a first terminal connected tothe first region and a second terminal connected to the input terminalof the charge pump, the second rectifying device is a second diodehaving an first terminal connected to the second region and a secondterminal connected to the input terminal of the charge pump; and whereinthe step of limiting comprises connecting a first terminal of a groundedzener diode to the output voltage of the charge pump such that a levelof the output voltage does not exceed a breakdown voltage of the zenerdiode.
 19. A method of claim 17 wherein the step of connecting the gateof the bidirectional MOSFET further comprises:connecting a firstterminal of a first zener diode to the charge pump and a second terminalof the first zener diode to the first region; and connecting a firstterminal of a second zener diode to the charge pump and a secondterminal of the zener diode to the second region.
 20. A method of claim17 wherein the step of connecting the gate of the bidirectional MOSFETfurther comprises:connecting a first terminal of a second MOSFET to thecharge pump and a second terminal of the second MOSFET to the gate ofthe bidirectional MOSFET; connecting a first terminal of a first zenerdiode to a gate of the second MOSFET and a second terminal of the firstzener diode to the first region; and connecting a first terminal of asecond zener diode to the gate of the second MOSFET and a secondterminal of the second zener diode to the second region.
 21. A method ofclaim 17 wherein the step of connecting the gate of the bidirectionalMOSFET further comprises:connecting a first terminal of a firstdepletion-mode MOSFET to the charge pump and a gate of the firstdepletion-mode MOSFET to the second region; and connecting a firstterminal of a second depletion-mode MOSFET to a second terminal of thefirst depletion-mode MOSFET, a gate of the second depletion-mode MOSFETto the first region, and a second terminal of the second depletion-modeMOSFET to the gate of the bidirectional MOSFET.
 22. A method of claim 17wherein the step of connecting the gate of the bidirectional MOSFETfurther comprises:connecting a first terminal of a first depletion-modeMOSFET to the charge pump and a gate of the first depletion-mode MOSFETto the second region; connecting a first terminal of a seconddepletion-mode MOSFET to a second terminal of the first depletion-modeMOSFET, a gate of the second depletion-mode MOSFET to the first region;connecting a first terminal of a third MOSFET to the charge pump, asecond terminal of the third MOSFET to the gate of the bidirectionalMOSFET, and a gate of the third MOSFET to a second terminal of thesecond depletion-mode MOSFET.